Diel and seasonal patterns of soil CO2 efflux in a temperate tidal marsh

Tidal marshes store large amounts of carbon; however, little is known about the patterns, magnitudes, and biophysical drivers that regulate CO₂ efflux from these ecosystems. Due to harsh environmental conditions (e.g., flooding), it is difficult to measure continuous soil CO₂ efflux in tidal marshes. These data are necessary to inform empirical and process-based models and to better quantify carbon budgets. We performed automated (30 min) and manual (bi-monthly) soil CO₂ efflux measurements, for ~20 months, at two sites in a temperate tidal marsh: tall Spartina (TS; dominated by S. cynosuroides) and short Spartina (SS; dominated by S. alterniflora). These measurements were coupled with water quality, canopy spectral reflectance, and meteorological measurements. There were no consistent diel patterns of soil CO₂ efflux, suggesting a decoupling of soil CO₂ efflux with diel variations in temperature and tides (i.e., water level) showing a hysteresis effect. Mean soil CO₂ efflux was significantly higher at SS (2.15 ± 1.60 μmol CO₂ m^-2 s^-1) than at TS (0.55 ± 0.80 μmol CO₂ m^-2 s^-1), highlighting distinct biogeochemical spatial variability. At the annual scale, air temperature explained >50% of the variability in soil CO₂ efflux at both sites; and water level and salinity were secondary drivers of soil CO₂ efflux at SS and TS, respectively. Annual soil CO₂ efflux varied from 287-876 to 153-211 g C m^-2 y^-1 at SS and TS, respectively, but manual measurements underestimated the annual flux by <67% at SS and <23% at TS. These results suggest that measuring and modeling diel soil CO₂ efflux variability in tidal marshes may be more challenging than previously expected and highlight large discrepancies between manual and automated soil CO₂ efflux measurements. New technical approaches are needed to implement long-term automated measurements of soil CO₂ efflux across wetlands to properly estimate the carbon balance of these ecosystems.

Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: a synthesis

Background

Sediments frequently exposed to dry-wet cycles are potential biogeochemical hotspots for greenhouse gas (GHG) emissions during dry, wet and transitional phases. While the effects of drying and rewetting on carbon fluxes have been studied extensively in terrestrial and aquatic systems, less is known about the effects of dry-wet cycles on N₂O emissions from aquatic systems. As a notable part of lotic systems are temporary, and small lentic systems can substantially contribute to GHG emissions, dry-wet cycles in these ecosystems can play a major role on N₂O emissions.

Methodology

This study compiles literature focusing on the effects of drying, rewetting, flooding, and water level fluctuations on N₂O emissions and related biogeochemical processes in sediments of lentic and lotic ecosystems.

Results

N₂O pulses were observed following sediment drying and rewetting events. Moreover, exposed sediments during dry phases can be active spots for N₂O emissions. The general mechanisms behind N₂O emissions during dry-wet cycles are comparable to those of soils and are mainly related to physical mechanisms and enhanced microbial processing in lotic and lentic systems. Physical processes driving N₂O emissions are mainly regulated by water fluctuations in the sediment. The period of enhanced microbial activity is driven by increased nutrient availability. Higher processing rates and N₂O fluxes have been mainly observed when nitrification and denitrification are coupled, under conditions largely determined by O₂ availability.

Conclusions

The studies evidence the driving role of dry-wet cycles leading to temporarily high N₂O emissions in sediments from a wide array of aquatic habitats. Peak fluxes appear to be of short duration, however, their relevance for global emission estimates as well as N₂O emissions from dry inland waters has not been quantified. Future research should address the temporal development during drying-rewetting phases in more detail, capturing rapid flux changes at early stages, and further explore the functional impacts of the frequency and intensity of dry-wet cycles.

River-floodplain restoration and hydrological effects on GHG emissions: Biogeochemical dynamics in the parafluvial zone

The parafluvial zone is frequently exposed to drying-rewetting cycles with critical consequences for the biogeochemistry of soil and sediment in river-floodplain landscapes. Upon restoration of the hydrological connectivity, substantial changes in biogeochemical processes are expected. The effects of water fluctuation on the magnitude of GHG emissions were investigated in the parafluvial zone of a restored river floodplain in Austria. Sediment composition, DOM quality and N₂O, CO₂, CH₄ fluxes were quantified during distinct hydrological periods (intermittent, desiccation and post flood) and along a hydrological gradient. The hydrological gradient ranged from non-flooded plots in the floodplain soil (used as reference plots after restoration), to rarely-flooded and frequently flooded sediment plots in the parafluvial zone. Enhanced biogeochemical turnover rates were identified during the intermittent period, when N₂O and CO₂ emissions peaked. In particular, the frequently flooded plots showed significantly higher CO₂ and CH₄ emissions compared to non-flooded and rarely-flooded plots. This indicates a strong effect of water level fluctuation on GHG emissions, with higher emissions occurring during transitional stages of drying and rewetting. Strong positive relationships were found between individual GHG fluxes, suggesting a tight link between C and N cycles. Both the C and N cycles are dependent on similar substrate characteristics that are governed by the quality of the DOM pool. Interestingly, drier sediments in the rarely-flooded plots were also active areas for emissions. This highlights the importance to include dry phases and sites in the overall C and N emission estimates of riverine landscapes. From the restoration point of view, N₂O emissions in the parafluvial zone did not differ significantly from the emissions in the reference plots, whereas CO₂ and CH₄ fluxes did. When making management decisions to restore connectivity, one needs to carefully consider the interplay between nutrient removal from water versus GHG emissions, to reach maximum environmental benefits.